The Perfect Coffee    Worksheet

G'day, it's Dr Karl here.

Coffee is one of the most popular drinks in the world. We have been sipping it since at least the middle of the 15th century - in what is today the country of Yemen.

So you'd think that by now we would have nailed the secret to making a consistent and high quality cup of coffee. But no, according to an international consortium, we have been doing it all wrong!

The consortium was an eclectic bunch consisting of baristas (yep, they're the folk who actually generate that delicious liquid for you), computational chemists and mathematicians from Australia, Switzerland, the United Kingdom, Ireland and the USA.

Now we are not talking about instant coffee, nor plunger or drip filter coffee. We are talking about the purest form of coffee, not adulterated by milk, sugar, or heaven help us, citrus. We are talking ... espresso.

, states that you begin with some 6.5 -- 7.5 grams of ground coffee. You then force hot water through this ground coffee, at a temperature of 86-90°C, and at a pressure of some 9 atm. Do this is for about 20-30 seconds and you should end up with around 23-27 mL of dark hot coffee liquid.Italiano Espresso IstitutoItaly is the home of the espresso, and the Italian definition of espresso, according to the

But your average coffee shop is not what you'd call a stickler for these guidelines. They start with more than double the amount of ground coffee (say 15-22 grams) giving a larger volume of steaming hot dark liquid (say 25-35 mL).

!can notBut while they don't necessarily follow these definitions, baristas are very particular about their methods, and they try really hard to deliver a constant high-quality espresso. But they

Consistency has always been a niggling problem. There seems to be something random popping up somewhere in the system.

Now while espresso-making sounds pretty straightforward, this system is actually very complex.

How old should the coffee beans be when you grind them, and how finely should you grind those beans?

How much ground coffee powder should you use, and how firmly should you tamp it down?

What temperature and pressure should the water be as it travels through the ground coffee, and for how long should you let the water come through?

That's a lot of variables.

Let's start by taking a close look at a single grain of finely ground coffee, nestled in between a whole bunch of other grains.

In total, these grains take up about 82% of the available space, which means that about 18% is free for the hot pressurised water to pass through. Each grain is around 100 µm in diameter -- roughly the width of a human hair. And within each grain of ground coffee are (at last count) some 2,000 recognised chemicals!

These chemicals have to leach out from inside a dry grain into the hot pressurised water being forced past it, and mingle with the rest of the coffee liquid before it ends up in the espresso basket - and then drip into your small cup.

If you wanted to solve the mathematics and the physics of the transport equations relating to the movements of these 2,000 chemicals into your shot of espresso, you would need "more computing power than Google has".

Luckily for our international coffee consortium - the modelling mathematics they needed had already been nutted out by another field of science, Electrochemistry.

You see, engineers and scientists have been trying to understand how lithium ions travel through the electrodes of a lithium battery -- and their equations were perfect for the coffee scientists.

Now you would think that the more finely you grind the coffee, the more coffee goodness and wonderful taste you would achieve.

But no, and this was a big surprise!

The consortium's research explained why a barista could always carry out exactly the same process, but end up with two espressos that tasted quite differently.

Here's the secret:

If the coffee beans had been ground too finely, the hot pressurised water forcing its way through would randomly force the coffee powder to coagulate into an almost solid lump. Very little coffee goodness would be extracted from those precious grains of coffee powder -- and correspondingly, lots of those 2,000 chemicals would be over-extracted from a nearby section of ground coffee as the hot water forced its way through the channels that it opened. This random clumping is why consistency has always been so hard to achieve.

According to the consortium, to make a more perfect and consistent espresso, we should grind the coffee beans less finely, use three quarters as much coffee, and run the water at a lower pressure -- six atmospheres, not nine.

Of course the proof is in the drinking -- and some baristas are saying that this new regime produces espresso that is better tasting and more consistent, from one cup to the next.

It's not exactly an independent double-blind trial, but after 600 years of taking a shot in the dark we might be on track for the perfect espresso every time.